Noncovalent intermolecular interactions, widely found in
molecular clusters and bio-molecules, play a key role in many
important processes, such as phase changes, folding of proteins
and molecular recognition. However, accurate calculation of
interaction energies is a very difficult task because the
interactions are normally very weak. Rigorous expressions for the
electrostatic and polarization interaction energies between two
molecules A and B, in term of the electronic densities, have been
programmed: (1) (2) Z is atomic charge, ƒâ0 is the electron
density of the isolated molecule and ƒ´ƒâind is the electron
density change of the molecule caused by polarization. With some
approximations, procedures for electrostatic and polarization
energy calculations were developed that involve numerical
integration. Electrostatic and polarization energies for several
bimolecular systems, some of which are hydrogen bonded, were
calculated and the results were compared to other theoretical and
experimental data. A second method for the computing of
intermolecular interaction energies has also been developed. It
involves a ¡§supermolecule¡¨ calculation for the entire system,
followed by a partitioning of the overall electric density into the
two interacting components and then application of eq. (1) to find
the interaction energy. In this approach, according to Feynman¡¦s
explanation to intermolecular interactions, all contributions are
treated in a unified manner. The advantages of this method are
that it avoids treating the supersystem and subsystems separately
and no basis set superposition error (BSSE) correction is needed.
Interaction energies for several hydrogen-bonded systems are
calculated by this method. Compared with the result from
experiment and high level ab initio calculation, the results are
quite reliable.

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